Two new lines of code added by nvm in .bashrc for explanation

after installing nvm, I found two lines of code in .bashrc: 

export NVM_DIR="/root/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"  -sharp This loads nvm

ask for advice, the second line of code . What does the preceding . in "$NVM_DIR/nvm.sh" mean?

Shell nvm
Mar.23,2021

. Equivalent to souce command

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