The receive capacity, or inbound liquidity, is the approximate amount of bitcoin that you can receive over a lightning channel.
Receiving bitcoin decreases your receive capacity, and increases your available balance. On the other hand, sending bitcoin decreases balance and increases your receive capacity.
From time to time, especially if you frequently receive more than you spend, you will be warned that a funding fee will be deducted from your incoming payment, because a new channel will be created for you.