What does this command do.

In the man page it is written that captoinfo converts a termcap
description into a terminfo description.

But wat is termcap description and a terminfo description.

Please give an explanation.

The captoinfo is the command which converts the termcap file to terminfo file.

For every model of the terminal it needs to tell the application programs what the terminal is capable of doing. It tells what escape sequences (or control characters) to send to the terminal in order to do things such as move the cursor to a new location, erase part of the screen, scroll the screen, change modes, change appearance (colors, brightness, blinking, underlining, reverse video etc.). These informations are stored in a file called termcap (terminal capabilities).

The termcap file format is a older version. It has some disadvantages like the termcap entries had a size limitation, the complicated format of the file, etc., So in order to convert it in to the new version we use the commandcaptoinfo which converts it in to the new format called terminfo.

The termcap and terminfo files differ mainly in the capability names and the entry syntax. Therefore, the captoinfo command only makes the syntactical transformations and vocabulary substitutions.