NTP is an abbreviation for Network Time Protocol. It's a large network of timeservers with stratum servers at the top of the hierarchy to provide you and your devices with the most accurate time possible.
As for what it does and why would you want it, it's a personal matter. Asuswrt-Merlin has a builtin NTP-client and server, which can be configured (limited) through the UI. ntpMerlin expands these possibilities.
I think the first post of ntpMerlin is self-explanatory. It tells you what it does and show screenshots of what options you can fiddle with and some cool graphs about how it's performing.
v3.4.5 Updated 2021-08-05 ntpMerlin implements an NTP time server for AsusWRT Merlin with charts for daily, weekly and monthly summaries of performance. A choice between ntpd and chrony is available. ntpMerlin is free to use under the GNU General Public License version 3 (GPL 3.0). This...
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Edit: please edit the prefix of your title to
ntpMerlin, thay way threads are easier recognizable and can be grouped per add-on as well if someone is looking for answers on a specific add-on.
(I just did it. -rm)