As the degrees of freedom become larger, the difference between the t and z distributions becomes

As the degress of freedom become larger, the difference between t and z distributions becomes smaller.

In statistics, Student's t-distribution is used when the number of data or size of the population is small.

The degrees of freedom is given by the size of population. It is the number of data less 1.

The larger the number of data, the more the t-distribution approaches to the z-distribution.

So, the larger the degrees of freedom, the larger the population, and the smaller the difference between t-distribution and z-distribution.