If your tyre has a big hole in it so the tube can be seen from the outside it needs to be patched. The easiest way to do this is with a tyre boot on the inside.
You can buy ready-made bike tyre boots that stick on the inside of the tyre. You can pre-make them by cutting tough fabric and coating it with contact cement and letting it dry. The edges of the boot need to be soft otherwise the ridge will wear a new hole in the tube. In an emergency you can use heavy scrap paper, a folded bank note or tough vegetable material like a leaf from a tree or a shrub. Anything to get you home.
Stitch the bike tyre
If the cut is big it may need a stitch or two to hold it together. Dental floss is strong and waxed and useful for this job.
If the bike tyre has a sidewall failure – maybe caused by a misaligned brake block – you may be able to ride a low pressure by inflating the tube to the size of the tyre then mummifying the section that will sit inside the failed section of tyre with carpet tape. Wrap a section of tube that’s longer than the split. Install the tube, mount the tyre and pump the tube up as hard as possible without enlarging the tyre split. Go carefully – especially if the bodged repair is in the front bike tyre – as it may fail again suddenly.