Ask for advice: Display:fixed is located under the main navigation bar.

recently I saw the following page while studying Bootstrap,. I don"t know how to fix a "Display:fixed" navigation bar under the main navigation bar in the following picture.
www.ibootstrap.cn/

clipboard.png

understanding is achieved through Class "avbar-fixed-top", but not "fixed" should be the positioning of the corresponding browser form?

Mar.03,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b32656-2bdf3.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b32656-2bdf3.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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