What is "Go" in cribbage?
"Go" is when the count is less than 31 and the player whose turn it is to play has no legal plays (all cards remaining would put the count over 31). That player says "Go", the other person scores one point, and then plays any remaining cards in their hand (without going over 31). 

The explanation at cribbage.org from rule 1.5.e.2 ...


  
  Players alternately play (place face up on the table) one card at a time, starting with the pone. The cumulative value of the cards played is announced as each card is played.
  When a player cannot play a card without the cumulative total exceeding 31, that player calls "go", and the opponent continues to play all cards possible (not exceeding a 31-count). The player who calls "go" plays first in the following 31-count sequence.
  Points are scored during this play of the cards (see scoring chart in rule 1.7).
  


Scoring occurs normally on these cards &ndash; points for pairs, runs, etc. are still scored &ndash; except that if such play takes the count to 31, that player scores an additional point: you get two for reaching 31 or one for Go, but not both.

See the definition here and the scoring rules below.

The impact is generally small. It's unlikely that you'd get much more than two points during this time, as it takes at least two cards from your hand to reach a state where the other person would call Go, leaving you only two to play. You might be able to score a triple, but more likely you'd just get two or maybe four (with a pair) ... and your opponent might get those points back if you end up playing your remaining cards, e.g. by having 4-5-6 and being able to play them consecutively (after you've played your last card following Go).