Closed list describes the variant of party-list proportional representation where voters can (effectively) only vote for political parties as a whole and thus have no influence on the party-supplied order in which party candidates are elected. If voters have at least some influence then it is called an open list.
In closed list systems the party has pre-decided on who will receive the votes for the political parties in the elections, that is, the candidates positioned highest on this list tend to always get a seat in the parliament while the candidates positioned very low on the closed list will not.
However, the candidates "at the water mark" of this specific party are in the position of either losing or winning their seat, depending on the specific total closed list votes for this party.
"The water mark" is defined as the number of seats a specific party can be expected to achieve, in reference to how the party produces their closed lists, that is, the few candidates who might or might not get a seat.
Voting systems using a closed list use a list of candidates selected by the party. Whoever controls this list is in a crucial power-brokering role. Those members elected from the list are essentially in thrall to the list maker--their political survival depends on how high up the list their name appears, or whether it appears at all. The party executive or party leader generally control the list, and closed-list voting systems transfer political power to the un-elected persons (strategists, delegates, party officials, etc.) who author the party's list of candidates.