What is the gap concept?

[Number Four]

I'm new to this forum (great info so far!) but I'm wondering if someone could briefly explain the "gap concept"?

[Eric]

you need a better hand to call a bet than to initiate a bet.

[Jeebus Kliest]

Essentially, you can raise into an unraised pot with a hand...and that same hand would fold to a raised pot.

Takes a stronger hand to call a raise than make one.