Error arduino ecg plotter

Does anyone know what's this on the top of my plotter??

This is the code I'm using for my ECG.

const uint8_t pinECG = A0;

void setup() 
{
    //don't need to declare A0 as input (default)
    Serial.begin( 115200 );
    
}//setup

void loop() 
{
    uint16_t ecg = analogRead( pinECG );
    float volt = (float)ecg * 5.0 / 1024;
    Serial.println( volt );
    
}//loop

A good start would be to configure the Serial Monitor & Serial plotter to use the same bitrate as the Arduino program.

Serial.begin( 115200 );

The thing at the top of the plotter is the key for the chart. It is intended to allow you to indicate what each variable represents (the plotter supports multiple variables).

This time it was populated with gibberish due to the baud rate mismatch, as lastchancename already explained, but perhaps you might like to use the feature intentionally some time. If so, you can learn about it here:
https://github.com/arduino/Arduino/blob/master/build/shared/ArduinoSerialPlotterProtocol.md#serialplotter-protocol

Note that there is an error in that documentation: Spaces in label text are not supported since spaces are one of the supported data separators.