Except for under very limited circumstances the Texas State Constitution does not allow the State Legislature to write laws that only apply to certain counties. Texans, in general, value local control and the framers of the state constitution wanted to keep lawmakers in Austin from meddling in local affairs too much.
Of course the constituents of those lawmakers still expect them to help out with local issues. So what's a lawmaker to do? They could work to pass laws that would address the local issue but would apply to the whole state, but the other state lawmakers may not want to subject their constituents to rules designed for another part of the state. It takes a majority of both houses to pass a law - so the author of a law designed to address a local issue must find a way to write it so that it will not affect anyone else's constituents.
The solution is a "bracket". A bracket is a way of describing the counties to which a law applies. Lawmakers can't write a law that says "this law only applies to county X" but they can write a law that only applies to a county that has certain characteristics.
By creating very narrow requirements a law can be written so that it only applies to one county.
In order to comply with the State Constitution the bracket must be written so that a county can grow out of or into, so you often see population being used in brackets. Other characteristics - such as containing a facility licensed for a space port, or being licensed for a low-level nuclear waste facility - also get used from time to time. Since a license can be obtained or revoked it fits the "grow out of or into" criteria.
See if you can figure out which counties are described by these brackets, taken from bills filed for the 82nd legislature.
SB 105: applies only to "a county that is located wholly or partly above a hydrocarbon-producing geological formation that underlies all or part of two counties: (1) that are adjacent to one another; and (2) each of which has a population of more than 1.4 million."
HB 54 applies only "In a county located on the Texas-Mexico border that has a population of 500,000 or more and is adjacent to two or more counties each of which has a population of 50,000 or more"
HB 107 applies only to "a county: (1) with a population of less than 185,000; and in which two or more municipalities with a population of more than 65,000 are wholly located."
State elected officials tend to give each other a great deal of freedom to deal with local issues. Most of the time if a lawmaker can bracket their legislation in such a way that it only affects their own constituents other officials will not object (not all of the time, of course). It's important, when you see a bracket, to ask yourself why the author wants the law to apply so narrowly. Sometimes brackets can be used to further political vendettas by overriding the decisions of county commissioners or city councils.
If nothing else attempting to decipher a bracket can be a fun education in the geography of Texas.