Recently, my school's administration announced some network changes. Should I want to use the school's WIFI, I'd have to download an SSL Certificate to my MacOS (High Sierra v.10.13.1) I'll admit that I have limited knowledge of what this all means. I'm afraid that this will enable them to access my private information and search history. Is that the case? Can they see what I am doing when I am not connected to their wifi through a SSL Certificate? Please excuse my ignorance :)
As alluded to by many of the comments on your question, the information you provided isn't enough to give a definitive answer, but here's my guess being that I am very familiar with typical campus infrastructure.
Your school likely has their own CA (certificate authority) that they use for various services on their network. Since they made the CA (and its not that widely used), your device doesn't recognize the CA and doesnt trust it. In order for your device to trust the school's CA, you need to install the certificate. Since you are required to install the CA certificate to use the wireless, my guess is your campus is using WPA2-Enterprise with RADIUS authentication using PEAP and MSCHAPv2 (pretty common). The P in PEAP stands for protected, in this case using a TLS (the new fancy term for SSL :P) to protect the communication being transferred. This does NOT give the university any direct access to your device whatsoever. This will merely allow you to connect to their network which may have monitoring ramifications (such as Deep Packet Inspection, location based on access point, Identifying your machine by the MAC address, etc) but this can occur on any netowrk regardless of the TLS certificate installation. The TLS certificate and CA install will not be adding any remote monitoring software or anything crazy like that.