Complex sentence: To check the accuracy of a carpenter's type level (i.e. whether the level indicates that a truly horizontal surface is, in fact, level), it is placed on a flat and roughly level surface and the reading on the bubble tube is noted.
Split into simpler sentences: To check the accuracy of a carpenter's type level, a perfectly horizontal surface is not needed. The level is placed on a flat and "roughly" level surface and the reading on the bubble tube is noted.