Ordinal data is used in parametric tests. Mnemonic: OP.
Scores are an example of ordinal data. In order to check which scores are higher and which lower, we need to rank data. We rank data by assigning numbers that correspond with their magnitude (i.e. 1 for the lowest value, 2 for the second lowest and so on).
Ranking the scores of the table below is easy.

Scores Rank 5 2 2 1 9 4 12 5 7 3
But what to do when some of the scores are equal? When some scores are equal we make tied ranks.
Tied ranks are ranks shared by more than one score.

Scores Rank 5 2 2 1 12 4 12 4 12 4
In the table above we have three 12s. The procedure to deal with tied ranks is the following.
1. Assign the 3 scores the average of the ranks they would have had.
So we do 3+4+5 = 12/3 = 4. So all the three 12s get a 4. Sometimes some tied ranks will have decimals. It is fine.
Note: any number following the 12 above would initially be assigned 6 because 3,4 and 5 were used in the ranking computation.