The word “gerrymandering” refers to the practice of drawing political district maps in strange ways, usually for partisan purposes. In other words, gerrymandering consists of drawing oddly-shaped districts to help your political party have a better outcome in elections.
In Texas, as in the rest of the United States, state legislators and US Representatives are elected from specific geographical districts. These districts have to have roughly the same number of people in them. This means that maps must be drawn creating the right number of districts, all with similar numbers of people. Ideally, the districts would be drawn in regular ways with each district being more or less the same shape. However, political parties prefer to draw lines in ways that will benefit them. Whichever party is in charge of drawing the district lines will try to draw districts in such a way that the most possible districts will have voters from their particular party. By drawing districts in this way, they give themselves an advantage that they would not have if districts were drawn in a fairer fashion.
In the map in the link below, we can see that some Congressional districts in Texas appear to be drawn in very strange ways. For example, the district in pink in the south of the state is long and skinny with many outcroppings. The district in navy blue in the north is shaped roughly like the letter “y.” These districts are not drawn in the way that you would expect them to be drawn. Instead, they and other districts appear to have been gerrymandered.