Short answer. When a person threatens another with a wrong amounting to a crime, aimed at the victim's person, honour or property, or at his family. The threatened harm must itself be criminal. The penalty depends on whether a condition or demand was attached and whether the offender got what he wanted.
What the law says
Any person who shall threaten another with the infliction upon the person, honor or property of the latter or of his family of any wrong amounting to a crime
Revised Penal Code, Article 282 — Grave Threats. Read the full provision →
The threatened act must be a crime
This is the dividing line people miss. Article 282 is not about rudeness, insults or intimidating tone. The wrong threatened has to amount to a crime — killing, burning a house, inflicting serious injury, and the like. Saying you will sue someone, report them to their employer, or stop doing business with them is not grave threats, however aggressively it is said, because none of those is a crime. The target can be the person threatened or a member of his family, and the harm can be aimed at the person, the honour or the property. Threats of a wrong that is not a crime are dealt with elsewhere in the Code as light threats.
With a condition, and without
The Code grades the offence by what the threat demanded. Where the offender made the threat demanding money or imposing any other condition, even though not unlawful, and attained his purpose, the penalty is the one next lower in degree than that for the crime he threatened to commit. If he did not attain his purpose, the penalty drops by two degrees. Note the phrase "even though not unlawful": the condition itself need not be illegal — demanding something you are actually owed, backed by a threat to commit a crime, still falls inside. Where no condition at all was attached, paragraph 2 applies instead, with a fixed penalty rather than one keyed to the threatened crime.
In writing, through a middleman, and the fine today
If the threat is made in writing or through a middleman, the penalty is imposed in its maximum period. Text messages, chat, email and letters therefore aggravate rather than soften the offence, and so does sending someone else to deliver the message. For an unconditional threat the penalty is arresto mayor and a fine not exceeding one hundred thousand pesos. That peso figure is the current one because of Republic Act No. 10951, enacted in 2017, which revised the fines and property values throughout the Revised Penal Code. Older reproductions still print the 1930 amounts, so treat any smaller figure you find online as out of date.
What to do on either side of it
If you have been threatened, preserve the evidence exactly as received — screenshots showing the sender and timestamps, the original device if possible, and the names of anyone who heard it. Report it to the police or go straight to the prosecutor's office; where the parties live in the same city or municipality, many disputes must first pass through barangay conciliation, though that requirement does not apply to every case. If you are the one accused, an angry message sent in the heat of an argument is not automatically this crime, but deleting it is a poor strategy. Either way the case is decided on the exact words and the surrounding facts, so get counsel to look at them before you make a statement.