Thanks for your question.
In the Old Testament, the usual word for "holy" is "kodesh" or a derivative of that word and it literally meas "set apart" or "separate."
The New Testament word translated "holy" is "hagios" or a derivative of that word and it also means literally to "separate" or to "set apart."
For example the phrase "holy Bible" literally means "separate book" or a book that has been separated as being special to God.
Another example is in the word spirit. God upholds the world and the universe by His spirit which is his power. His "Holy Spirit" is His power focused or separated to perform a specific task. You can think of it like the light from the sun which is everywhere present during the daytime on earth - that's like the spirit. However, if you take a magnifying glass and focus that sunlight to start a fire in a particular spot, that's like "Holy Spirit".
I hope you have found this helpful.