“2,4,5-T” (short for 2,4,5-trichlorophenoxyacetic acid) is a synthetic herbicide.

• It was used to kill broadleaf weeds and shrubs, especially in forestry and agriculture.
• It is chemically related to 2,4-D, another herbicide.

Important note:

• 2,4,5-T became controversial because its manufacturing process could produce dioxin (TCDD), a highly toxic contaminant.
• Due to health and environmental concerns, it has been banned or heavily restricted in many countries since the 1970s–1980s.

In short: it’s a now largely banned weed-killing chemical due to toxicity concerns.