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 we speak of 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" 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.