Punched cards, like punched paper tape, magnetic tape & magnetic drums were all forms of data storage in the early development of digital computers.
They served a similar function as magnetic hard drives, solid state drives, memory cards & USB memory storage devices currently do.
They store data and software (programs, applications) that can be accessed and used by computers.
With punched cards, each card represented one line of information - analogous to one line of a text file, one line of code in a computer program or one line of data from a data file. Each column on the punched cards represented characters, or numbers, depending on which holes in the column were created.
Most computer cards had holes numbered from 0 to 9 going down the card & holes 11 and 12 at the top of the card (hole 12 being the topmost hole) - particularly those that used the DEC 029 code for punching holes.
With punched cards, only capital letters were used - due to limitations imposed by computer memory and how many bits were in a byte.
Under the DEC 029 code, the letter A was designated as holes 12 & 1 in a column. The letter B was holes 12 & 2. The letter J was holes 11 & 1. For special characters such as + - = < >, three holes would be punched in a column, such as 0 3 8 (me reviving long dormant brain cells).
For numbers, the number 1 was hole number 1 and number 5 was hole number 5, etc. To resent a number like 24, one column would have only the number 2 hole punched and in the adjacent column to the right, the number 4 would be punched.
The way punched card readers worked is each card would be placed against a shiny metal plate and the card was illuminated by a strong light. The metal exposed by the holes would reflect the light which would then be read.
For people who did not have access to a punched card machine to punch the holes into the card, the holes could be manually marked with a HB pencil - preferably using a large dot filling most of the rectangle. The graphite in the pencil lead reflected light in a similar way as the metal backing when the cards had holes.
Edit - several weeks later
Computer cards had eighty columns of holes, which is why a lot of old computer programs, from the 1960s and 1970s was limited to having eighty characters per line.