Ubuntu – How to do Kubernetes pod communication

I learning Kubernetes and trying to deploy NodeJs app. How can I connect my MongoDb database to backend manifest files? this is my nodejs i.e. backend logs.

root@ip-172-31-26-165:/home/ubuntu# kubectl logs backend-59666ccd6b-ltbbs
Server is running on port: 3000
MongoDB connection failed!!! MongooseServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
    at Connection.openUri (/app/node_modules/mongoose/lib/connection.js:847:32)
    at Mongoose.createConnection (/app/node_modules/mongoose/lib/index.js:291:17)
    at connectDB (/app/config/database/mongodb.js:5:14)
    at Object.<anonymous> (/app/config/database/mongodb.js:13:29)
    at Module._compile (node:internal/modules/cjs/loader:1112:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1166:10)
    at Module.load (node:internal/modules/cjs/loader:988:32)
    at Module._load (node:internal/modules/cjs/loader:834:12)
    at Module.require (node:internal/modules/cjs/loader:1012:19)
    at require (node:internal/modules/cjs/helpers:102:18)
    at Object.<anonymous> (/app/server.js:4:17)
    at Module._compile (node:internal/modules/cjs/loader:1112:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1166:10)
    at Module.load (node:internal/modules/cjs/loader:988:32)
    at Module._load (node:internal/modules/cjs/loader:834:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:77:12) {
  reason: TopologyDescription {
    type: 'Single',
    setName: null,
    maxSetVersion: null,
    maxElectionId: null,
    servers: Map(1) { 'localhost:27017' => [ServerDescription] },
    stale: false,
    compatible: true,
    compatibilityError: null,
    logicalSessionTimeoutMinutes: null,
    heartbeatFrequencyMS: 10000,
    localThresholdMS: 15,
    commonWireVersion: null
  }
}