What does tbl mean and how is it done. This is part of a cable stitch pattern

check out this YouTube video - watching is the best illustration sometimes.

not sure if you are talking about knit or Purl … but you can google
"K tbl" video or “P tbl” video

Hope this helps

In the Glossary here at knitting help, there’s entries for both knit and purl tbl - through the back leg - and videos for both.