There are many things that can cause brown spots on basil. I cannot say anything for certain based on your picture. Too much or too little water, insects, and diseases can all cause problems. One of the most common problems with basil is downey mildew. To look for this check the underside of the leaves and look for gray fuzzy appearance. I do see spots where it looks as if insects may have been eating. Check the plant closely including the back side of the leaves and on the stem for insects. You may need to check at night as well as some insects feed at night. Is the plant in the ground or in a container? Water can be a problem as well. We have gotten a lot of rain recently. If there is not good drainage this could cause brown leaves. Here are some bulletins with more information on these problems and identifying them.

https://extension.umn.edu/vegetables/growing-basil

https://plantvillage.psu.edu/topics/basil/infos