LastPass installs browser extensions for any browser (Chrome, Firefox, Safari, etc.). When you sign into an account that LastPass doesn't know, you will be asked if you want LastPass to remember the information:
Test your current "go-to" password (or something similar to it to be safe) at howsecureismypassword.net to see how long it would take a computer running through every combination of characters to reach a matching sequence. Try out the different between "mittens" vs. "7y!jr5k9_mls32". One is easy to remember and one is uncrackable, but a password manager allows you to have an option of which one you want to use. And your master LastPass password allows you to look up that autogenerated password anytime you need to.
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Have a great weekend!