TL stands for Trust Level, it’s mostly an automated process of which forum users gain more abilities on the forum over time. This is to make a gradual and simple introduction of the forum possible and make moderation easier.
TL1 can be reached by spending about 15 minutes on the forum. TL2 can be reached by spending about 15 days on the forum. This includes liking and commenting. The statistics are cumulative, so there is no need to make this a daily routine. It’s also possible for staff to manually promote users. You may request a manual promotion in this thread.
More information about the criteria of these trust levels and what they allow you to do:
The table below gives a very detailed overview of what each trust level allows you to do.