As mentioned by others, 'tho' is the informal version of 'though'.
Here are some examples from an actual text conversation I had with one of my friends:
Idk who's driving tho
I prob can't come to dinner tho
I haven't gotten that far in my search tho lol
In all instances, 'tho' can be replaced with 'though' but in texting, it is faster to type 'tho'.
Some other examples of informal words from the sentences above are:
Idk = I don't know
prob = probably
lol = laughing out loud