You will find in some manuals , the meaning of the symbols are written at the bottom of the manual's page. But it is in Japanese , which some of us can't understand.
Therefore , i will show you the symbols and tell you what it means :D
Symbol : A brush.
Meaning : Apply glue here.
You seldom find this in manuals of kits nowadays because Bandai has develop to a level where kits can be built without using glue now. :D
Symbol : White circle BIGGER than black circle.
Meaning : Assemble/install this section first.
Symbol : white circle SMALLER than black circle.
Meaning : Assemble/install this section last.
Symbol : Peel up layer with Japanese label.
Meaning : Apply STICKER (number/alphabet) here.
Symbol : A pointy object pointing at a black dot with label.
Meaning : Apply DECAL (number / alphabet) here.
Symbol : Cutter / Nipper.
Meaning : Cut this part.
Symbol : Curve arrows pointing in two opposite directions in a half black half white box.
Meaning : Move a particular part , as well as the opposite part.
This symbol you will usually see it in most of the MG kits when you are assembling the waist , where the part which joins the legs.
Symbol : Screw driver with exclamation mark ( ! )
Meaning : Beware when tightening part with screw. Do not over tighten the screw.
Symbol : Arrows pointing at opposite side , each side with negative color(black / white) with part label.
Meaning : Part labelled is to be installed on the opposite side.
Symbol : Exclamation mark ( ! ) in a black box.
Meaning : Pay attention to this part.
Symbol : X2
Meaning : Repeat procedure twice.
Symbol : Arrow in circle motion with degree label.
Meaning : Rotate the particular part as indicated (90* / 180* / 360*)
This is some Mathematics.
You may leave a comment below with your email to ask me about this. :D
Symbol : Two arrows point at a white bar in the middle.
Meaning : Install identical / same part for the opposite side.
Symbol : Circle and Triangle.
Meaning : Select / Choose a piece of two (or more) part given and assemble.
For example , MG Zaku Shin Matsunaga , you may choose between , the Stabilizer horn or the Normal horn for the head.
Symbol : Character (前) with arrow point at a direction.
Meaning : The front of the part / piece facing the direction according to the arrow.
(前) means front in Chinese , which i guess is the same in Japanese.
Symbol : Number in black diamonds.
Meaning : Install parts according to the number in ascending order. (1>2>3>4> and onwards)
This is also some Mathematics xD
May leave your email in the comment section bellow to ask if you are confused. xDD
Symbol : Japanese writing. (like sound expression you see in manga/Japan comics)
Meaning : Parts will be snap fitted correctly once you hear sound of pieces snapped together.
The writing probably means the sound of "Pak" or "Tac" , as long as there is sound xD
I hope that this will help you with your building ^^
If i have left out any symbols , or you would like to ask more about the symbols , please leave a comment below ^^
Enjoy your Gunpla ! xD