This is the 38th most frequent Japanese word.
第
"第" is a prefix meaning "number," "order," or "sequence" (e.g., first, second).
In this sentence, '第' is used as a prefix to denote an ordinal number (the first episode).
第1話はとても感動的でした。
Episode 1 was very moving.
Here, '第' precedes a positional rank, indicating the highest rank or position (1st place).
次の大会で第1位を目指します。
I aim to achieve 1st place in the next tournament.
In this context, '第' is used to indicate a sequence stage (the third stage).