Tally marks are a system of recording (usually) small numbers.
For each unit in a given natural number, a mark is made.
Thus, for example, $13$ could be recorded $|||||||||||||$.
It is usual for tally marks to be arranged in groups of $5$, by drawing every fifth line diagonally across the group of $4$ before it.
Thus $13$ would then be recorded:
Tally marks are the earliest known records of numbers.
However, the technique is still useful, for example, when recording the number of instances of events of irregular frequency.