namark/libsimple_motion: what even is a tween? @ 6db2dc974a27d1fafc33201942b6b3a33d8c8fa9

what even is a tween?

namark 6db2dc974a allinclude		10 ヶ月前
source	6db2dc974a allinclude	10 ヶ月前
unit_tests	2dbc37866b Default constructed melody considered empty/done.	4 年前
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Makefile	564130634c Initial state extracted from sketchbook.	4 年前
README.md	564130634c Initial state extracted from sketchbook.	4 年前

what even is a tween? shorthand for between?? what even is between???