Altcoins and Bitcoins tend to react to each other. Sometimes they do the opposite of each other and sometimes they do exactly the same thing. It is not rare to see Bitcoin go down while alts go up (and vice versa). This is because almost everyone who has alts has Bitcoin, so they tend to move out of Bitcoin when it goes down and move into alts (and vice versa). Almost just as often as this is the case it isn’t the case. Many times, all coins will go up or down together (generally following Bitcoin’s lead). This dance often results in Bitcoin outperforming altcoins, however every x months we will see an alt boom where alts outpace Bitcoin quickly. If you can time that, great. Try to spot it coming and there is big money to be made. Meanwhile, alts can be tricky to just HODL, as they tend to lose value against fiat and BTC in the off season. Learn more about the relationship between Bitcoin and Alts. In a word, alts are generally more volatile than Bitcoin.
Always pay attention to Bitcoin. Most altcoins (every cryptocurrency except Bitcoin) are pegged more closely to Bitcoin than Asian currencies were to the USD during the Asian Financial Crisis. If Bitcoin price pump drastically, altcoins price can go down as people try to exit altcoins to ride the BTC profits; inversely, if Bitcoin prices dump drastically, altcoin prices can go down, too, as people exit altcoins to exchange back into fiat. The best times for altcoin growth appear when Bitcoin shows organic growth or decline, or remains stagnant in price.