In corpora such as IG each Inscription has its own number and a lemma. The lemma contains brief information on the form, findspot, present location, previous publications etc. and is normally provided with some notes. Reading a lemma of IG or another corpus is necessary, but may seem unnecessary difficult as they are often in Latin. (This is the subject of longstanding debates in epigraphy land)
The IG lemma normally starts with the inscription number, followed by the number from a previous IG edition; then the site and current location. A brief description of the monument, material and dimensions follow, and information on the shape and dimensions of the letters. This is followed by the earlier editions. The abbreviation ect(typum) means that there is a squeeze, i.e. a paper print, in Berlin. Furthermore, dating and text (the vertical line | indicates a line break on the stone), followed by a short commentary, literature references and references to other relevant inscriptions in IG.
IG and CIL have provided their own editors with helpful glossarie. You will find them here. Please note - the documents are FTM Latin-German only. I am working on a translation. Until then: use DEEPL for the German,