1. A term referring to the amount of you expect to gain on average if you make a certain play. For instance, suppose you put $10 into a $50 pot to draw at a hand that you will make 25% of the time, and it will win every time you make it. Three out of four times, you do not make your draw, and lose $10 each time for a total of $30. The fourth time, you will make your draw, winning $50. Your total gain over those four average hands is $50-$30 = $20, an average of $5 per hand. Thus calling the $10 has a positive expectation of $5.
2. The amount you expect to make at the poker table in a specific time period. Perhaps in 100 hours play, you have won $527. Then your expectation is $5.27/hr. Of course, you won't make that exact amount each hour (and some hours you will lose), but it's one measure of your anticipated earnings.