This entire answer is probably a spoiler.
In short, communication is the use of signs to convey meaning. The book is a collection of progressions in inter-species communication achieved through signs and symbolic actions, almost all of them non-verbal. I suspect the title itself is meant to be thought-provoking in a way that naming the book "communication" wouldn't be!
Communication with the vines was via planting or maintaining the friendly vines, and planting against the unfriendly vines. With no direct communication possible, those were the only signs that could be used to communicate.
Communications with fippolions and fippobats was simple and involved both words and actions:
"Pitman!" I kicked hard at a bridge railing and howled. I
hoped his tiny [Fippolion] brain would see and understand: destroy the bridge.
Communication with the bamboo started out just as simply as the vines - doing what would make the bamboo prosper, and receiving bounties as a result. But it advanced when the bamboo began calculated displays designed to communicate:
Along the road leading to the riverside gate, the leaves and stems of
thigh-wide, sky-high stalks of bamboo had changed color, one stalk per
color on either side. Red, orange, yellow, green, cyan, indigo, and
violet. The bamboo had re-created a rainbow on either side of the
Sylvia clearly states how this sign is intended to communicate meaning:
"We all know that the bamboo has become much more healthy with us
here. We've given it water and fertilizer, and it's given us fruit
and keeps improving it. Now, the bamboo seems to want our attention.
I propose appointing Higgins to communicate with the bamboo for us."
And the bamboo attempts to communicate with more complex signs:
The next day, the bamboo flower buds opened, smaller and not as showy
as the others, half of them white and the other half black.... The
white flowers pointed up, the black ones pointed down. The thistles
at the base of the culms with the black flowers died, white flowers'
thistles lived.... The nectar tasted different, one acidic and one
which, as described by the bamboo, are ambitious in their meaning:
No response. Did they fail to understand? Dualism. The universe
consists of fundamental and antagonistic entities and forces. Animate
and mineral. Plants and animals. Parasites and producers. Creation
and destruction. Acids and alkalis. Sickness and health. Sky and
Answer me! Dualism is a simple idea. Light, dark. Up, down. Live,
dead. Communication, silence.
Even if you do not understand, show me that you wish to
communicate.... A small action will suffice. Speak to me.