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Serial works both ways. You can type into the Monitor and your board will read it, letting you send commands from your keyboard. ⌨️

First check if data has arrived with Serial.available(). It returns how many bytes are sitting in the buffer, ready to read.

Reading when nothing is there gives junk. So you guard your read with available() and only act when real data exists.

Grab a single incoming character with Serial.read(). It pulls one byte from the buffer and hands it to you.

Now branch on what you got. Compare the character and run different actions, like turning an LED on or off.

Notice the single quotes around '1'. That is a character, not the number one, and matching it correctly matters.

For full commands like on or off, read a whole line with readStringUntil(), stopping at the newline you send.

Then test the whole word. If the String equals on, do one thing; if it equals off, do another.

In the Monitor, pick a line ending like Newline. It tells readStringUntil where each typed command stops.

Good feedback feels great. After acting on a command, println a confirmation so you know the board heard you.

You just built a tiny command interface. Type, the board reads, acts, and replies. That is real two-way control. 🚀

Which function tells you whether typed data has arrived and is waiting to be read?

You made the board listen: checking available(), reading characters or full lines, acting on commands, and confirming back. Two-way Serial done! 🎉