Text to binary
Every character is stored as one or more bytes, and each byte is shown as eight 0s and 1s: H is 72, which is 01001000, so Hi is 01001000 01101001. English letters, digits and punctuation take one byte each, the same as in ASCII. Other characters use UTF-8, so é takes two bytes (11000011 10101001) and an emoji four.
Binary to text
Paste binary into the Code box with a space or a comma between bytes, or as one unbroken run whose length divides by eight. Bytes with spaces between them may drop their leading zeros, so 1001000 1101001 also reads Hi. Anything other than 0 and 1, a run that does not split into eights, or bytes that do not spell UTF-8 text is marked with the reason.
Questions
Can it turn a number into binary?
No. It shows the bytes that store text, so the character 5 becomes 00110101, its code 53, not 101. To convert the number itself, use a calculator's programmer mode.
Is this ASCII or UTF-8?
UTF-8, which matches ASCII for the first 128 characters, so plain English text gives the same binary in both.