If you were to translate word for word into English Tha basically means "am" or "are" or "is". You also have 'S e and 'S ann, which also take the place of am. These form the Gaelic equivalent of the copula.
The difference between the three is if you are describing or identifying something. If you are describing something, you use tha. If you are identifying something you use 'S e or 'S ann (because they are emphatic and by identifying something you are emphasising what it is in Gaelic).
Tha am bòrd mòr - the table is big
'S e bòrd a th'ann - it is a table
'S e is used when you are emphasising a noun and 'S ann is used when emphasising anything else.
For example you would use the phrase 'S ann in the sentence
'S ann air a` bhus a chunnaic mi Iain - On the bus, I saw Iain.
About the "th": Like English, Gaelic uses the Latin alphabet in its own way. In English, "h" is put after "c," "s," and "t" to represent other sounds. Gaelic also uses an "h" after other consonants -- b, c, d, f, g, m, p, s, t -- to change the way they are pronounced. Here are a couple of helpful resources for pronunciation: https://en.wiktionary.org/wiki/Appendix:Scottish_Gaelic_pronunciation https://youtu.be/fnOKl5EPFtc
i'm glad someone brought up "bòrd mòr" in the comments... i just encountered it as such: "seo bòrd mòr" and the answer was "this is a big table". i answered, "this big table". a previous lesson went this way: "an gàrradh snog" (the nice garden) and "tha an gàrradh snog" (the garden is nice). do "seo" and "tha" operate the same way, implying "something is"--with "seo" just specifying "[this thing here] is"?