+∞ means positive infinity. (In layman's terms, it is an infinity with a positive measure. )

-∞, which means negative infinity. (A quantity takes negative infinitesimal, and its absolute value is infinite)

∞, which means infinity. The absolute value of a quantity is infinite, whether positive or negative.

These three symbols are concepts in the limit, and you learn to connect in the process. Studying advanced mathematics in college will deepen your study. One or two sentences are unclear. I suggest taking a book on advanced mathematics and starting from the first page. You will understand after reading the first chapter.