Hieroglyphs are written characters that consist of pictures. Such an image can stand for a sound, a syllable or a whole word. The ancient Greek word "hieros" means "sacred", "glyph" is something that has been engraved. So they are sacred signs, usually carved or chiselled into stone. The ancient Egyptians themselves called their writing the "writing of the words of God".
The ancient Egyptians were already writing with hieroglyphs over 5000 years ago. Originally, it was a pure pictographic script with about 700 characters. Later, symbols for consonants were added. So you had to think up the vowels yourself. If two sequences of consonants could be read in different ways, there were also auxiliary characters. This is like writing "Wrt" in German. This can mean "word", "value" or "host".
The decipherment succeeded in 1822, which means that since then we know what the signs mean. Napoleon's soldiers had found the Rosetta Stone. On it, the same text is written above in hieroglyphics and below in ancient Greek, which was known at the time. This made it possible to decipher the hieroglyphs.