low points in instrument tubing

It refers to points which are the lowest with respect to gravity. They may be at the end of a run (not usually bad) or in the middle (often bad). Liquid will remain in a low point unless there is a drain, and sediment will gather there. In natural gas lines in the US, we are required to deliberately put in low points (in parallel to the flow path) to catch and hold water and dirt so that they do not enter burners.

Be sure that your instrument air is clean and has a low dew point and low point problems will be few and far between.