If by Hot Topics you mean the Posts under Hot Tab here is the Answer -Posts that have had good interactions (i.e. upvotes and comments) and earned decent amount of money (the amount might vary between SBD 1 to SBD 50 or more depending on the category the post is in) in relation to time. What is meant by in relation to time is, the quicker your post makes money and gets interaction, the hotter it is and the higher chance you will have to be higher on the *Hot tab

If by hot Topics you mean which Tags earn the most here is the Answer-
Some Hot Tags based upon the Payouts on steemit are the following -
art,bitcoin,blockchain,blog,busy,cn,contest,crypto,cryptocurrency,food,funny,health, introduceyourself, kr, life, money, music, nature , news, photography, science,spanish, steem, steemit, story, travel, utopian-io, video, writing.