A Meeting of the Church

When a “local institutional church” meets together, what does that mean specifically?
Are they “worshipping?” Are they “having fellowship?” Are they “obeying the scriptural command to do so?" What exactly are they doing?
Can a meeting or fellowship with just one other professing Christian be considered a “church meeting?" For example, when two believers meet together for fellowship, is this a meeting of Christians and thus “a meeting of the church?” Didn’t Jesus say, "For where two or three are gathered together in My name, there am I in their midst."
(Matthew 18:20) What did He mean by that?