For the purpose of job interviews (and not necessarily any kind of sound definition), this means that the position you are looking at will have you dealing with software that implements some operating system services.
To give you some examples: you might be working on an SDS product (software-defined storage), perhaps a filesystem, or maybe a network-attached block device etc. that operating system has a way of interacting with and exposing to other software as it's own service. To make it even more concrete: you can write a filesystem in a way that it interfaces with Linux kernel, and, once installed, users will interact with it through VFS interface (so, they don't even have to know they are interacting with your specific product). ZFS is a good example of this kind of software.
Similarly, it could be SDN (network), eg. various VPN software s.a. OpenVPN. Or a users / permissions management software (eg. LDAP server, s.a. slapd). Software to manage system services (eg. systemd). Software to manage specialized hardware (on top of drivers) that, again, provides its services through the operating system (eg. nvidia-smi). Or, maybe it's a software that manages memory, terminals, virtualization, containers (eg. DMA over IB, tmux, QEMU, containerd).
Sometimes this category is extended to system monitoring or higher-level management tools that still give some kind of general service similar to the one provided by the operating system. Think about SAR or mdadm kind of tools. Or, this could also be testing of operating system services (eg. FIO).
Sometimes, this can also mean some kind of administrative tools that perform either more high-level management of operating system (eg. yum, the RHEL package manager), or management of groups of operating systems. Cloud tools can fall into this category (eg. boto3 AWS client). But, this is kind of stretching the definition.
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So, as per usual in the field of programming... we don't have an authoritative source and a definition for a widely used term. You just have to accept (at least for now), that different people mean different (but often significantly overlapping) things when they use the term and ask more questions to refine what they mean by it.
To give you some examples: you might be working on an SDS product (software-defined storage), perhaps a filesystem, or maybe a network-attached block device etc. that operating system has a way of interacting with and exposing to other software as it's own service. To make it even more concrete: you can write a filesystem in a way that it interfaces with Linux kernel, and, once installed, users will interact with it through VFS interface (so, they don't even have to know they are interacting with your specific product). ZFS is a good example of this kind of software.
Similarly, it could be SDN (network), eg. various VPN software s.a. OpenVPN. Or a users / permissions management software (eg. LDAP server, s.a. slapd). Software to manage system services (eg. systemd). Software to manage specialized hardware (on top of drivers) that, again, provides its services through the operating system (eg. nvidia-smi). Or, maybe it's a software that manages memory, terminals, virtualization, containers (eg. DMA over IB, tmux, QEMU, containerd).
Sometimes this category is extended to system monitoring or higher-level management tools that still give some kind of general service similar to the one provided by the operating system. Think about SAR or mdadm kind of tools. Or, this could also be testing of operating system services (eg. FIO).
Sometimes, this can also mean some kind of administrative tools that perform either more high-level management of operating system (eg. yum, the RHEL package manager), or management of groups of operating systems. Cloud tools can fall into this category (eg. boto3 AWS client). But, this is kind of stretching the definition.
----
So, as per usual in the field of programming... we don't have an authoritative source and a definition for a widely used term. You just have to accept (at least for now), that different people mean different (but often significantly overlapping) things when they use the term and ask more questions to refine what they mean by it.