Why FT8 and other WSJT-X modes report signal strength as a signal-to-noise ratio in decibels, referenced to a standard 2.5 kHz bandwidth (about the width of an SSB channel). A report of +3 means the decoded signal measured 3 dB above the noise in that reference bandwidth. Because FT8 occupies only about 50 Hz, decodes are routine at negative numbers, typically down to around -21 dB, and reports above about +5 are rare.
Watch out The S-meter style answers are the trap: FT8 reports are not S units at all, and nothing like "S3" or "3 dB over S9" is exchanged. Those belong to voice and CW RST reporting.
FT8 numbers are dB SNR in 2.5 kHz, not S units. Negative is normal; +3 is a strong one.