Normally, a server that monitors your network pulls info from your network devices (GET operation), though a read-only SNMP account on the switch (or whatever device). The switch can also by itself trigger a message to a server, this is called SNMP Traps. The switched can also be managed though SNMP (SET operation, configuration changes are made though SNMP, usually by scripts or management software).
The MIBs are installed on the server to interpret the SNMP info given by the switch, like what info is coming in, what units are used and so on.
From a security perspective, avoid SNMP v1, version 2c may be acceptable and v3 is preferred.