They are labels applied to groups of input and output pins. I forget which chip you decided upon. Download its datasheet from Microchip.com, and you will see the names. Using the 16F877 as just an example, its datasheet, page 9, shows the ports. Compare that to the pin numbers and port names on page 5.
The port pin identifications for PortB, e.g., RB1, RB2, RB3, etc. will be assigned to sequential pins, but that sequence can go around a corner of the chip. The supply pins are not given a port numbers. Some other pins, e.g., MCLR, and the pins used for an external oscillator may or may not be included in a port, depending on the chip.
From this question, is it safe to assume you are now studying programming? Which language?
Forgot to add that putting all like-kind inputs/outputs on one port, say the eight fine divisions of your caliper, can be an advantage when you go to read their state(s), as it can be done all at once. The alternative of reading some pins on one port and other pins on another port to make an 8-bit byte of data is a little more complicated and takes more time.
John