Joining tables

Hi,

I have 2 tables that I need to join through an intermediary table; really not my area of expertise so was hoping someone could put me on the right track. The tables are as follows:

Table 1 has the column 'test_run_id'

Table 2 has the column 'tag_id'

Would I be able to create the intermediary table using UNION, or do I need something else?

It'll be 1 test run id to many tag ids, for reference.

Apologies if I butchered any terminology!